The present disclosure relates to a liquid cell for microscopy and, more specifically, for atomic force microscopy (“AFM”) imaging. In particular, the present disclosure is directed to a liquid cell such as a microfluidic cell for AFM measurement, which provides an in situ sample (e.g., a membrane) characterization during filtration.
AFM is a type of high resolution scanning probe microscopy, with resolution to the order of fractions of a nanometer. AFM is used to image sample surfaces at nanoscale, in different chemical environments, such as water, specific solutions, and buffers. Typically, imaging in a liquid environment is accomplished with a liquid cell. In a liquid cell, the AFM probe and the sample are immersed in a small volume of liquid. In this manner, the liquid cell enables AFM to measure the sample surface characteristics such as the surface morphology, surface potential, hydrophobicity, and conductivity in a sealed liquid environment.
Among many applications of the AFM liquid cell, observations of membrane fouling, foulant deposition, and membrane integrity (e.g., changes in physical, mechanical, and chemical properties) are particularly important for gaining mechanistic insight into interfacial and chemical processes that take place at nanoscaled membrane surfaces. AFM detects a force of interaction between the surface of a sample and a cantilever probe held near the surface of the sample.
Membranes are scanned with AFM before or after filtration experiments. After filtration in a membrane module, the membrane sample is transferred to an AFM liquid cell for AFM imaging. Such transfer may change the membrane integrity and the chemical environment. As a result, the ex situ measurement may reveal limited or no information of dynamic changes of membrane properties (e.g., deposition of particles or chemical foulants, substance distribution, and surface zeta potential changes) and may even compromise the measurement accuracy due to sample contamination or damage. Also, current commercial liquid cell designs contain a static fluid provided by pumping or injecting a reagent to the liquid cell, which does not simulate the practical hydrodynamic conditions during filtration.
Accordingly, there is a need for a well-designed liquid cell to allow real-time and in situ observations and measurements of membrane properties and surface characteristic evolutions during filtration, fouling, and/or aging processes.